Mechanical movement



1965 R. D. HANSON 3,198,024

MECHANICAL MOVEMENT Filed July 12, 1963 IN VEN TOR. P/CHHRD D Hfi/vso/v United States Patent 3,198,024 MEQHANHZAL MGVEMENT Richard 1). Hanson, Detroit, Mich., assignor to Burroughs Corporation, Detroit, Mich, a corporation of Michigan Filed July 12, 1963, Ser. No. 2%,625 12 Claims. (Cl. 7 4 125) This invention relates to a mechanical stepping movement and, in particular, to an improved incremental stepping movement having a ratchet wheel which is advanced at a very rapid rate through the intermittent action of a controllable interposer.

This invention is particularly useful for the rapid feeding of business forms, tapes, punch cards, and the like. These information media are frequently fed incrementally. Business forms may be advanced a line at a time, tapes may be moved a block at a time, and punch cards may be stepped a column at a time. It is common for most incremental feeding mechanisms to be activated periodically. Some operate aperiodically, whereas others combine both modes by being intermittently operable. The present invention is of the latter intermittent type and thereby can accommodate a complex programmed feeding operation as well as a simpler manual operation.

Many devices have been proposed in the prior art for applying an incremental motion to a driven element. Generally, these mechanisms function satisfactorily at low speeds; however, some prove to become unsuitable as the speed of operation increases because of their excessive size, weight, and complexity. Excessive size and weight give rise to high static and dynamic inertia which must be overcome before and after each incremental feeding operation. Not only do the size and weight retard the rapid operation, but they necessitate the employment of a powerful actuating means which further tends to in crease the over-all size, as well as the expense, of the mechanism.

Some devices, because of their inherent complexities, are subject to a decrease in reliability as the speed of operation increases, due to structural fatigue and wear of their rapidly engaging parts. Other more recent high speed mechanisms incorporate electronic circuitry, which by necessity involves special installation and maintenance considerations and their associated cost.

Accordingly, it is one object of this invention to increase the speed and reliability of an intermittent stepping movement while retaining strength and simplicity of structure and mode of operation.

Another object of the invention is to improve a high speed incremental drive, by employing a triggering means,

which operates under only a slight work load and there-' by is able to respond rapidly either continuously or selectively.

Still another object of this invention is to simplify a fast acting driving mechanism by having a controllably actuated interposer which is repositioned by coaction with a ratchet pawl.

The present invention is adapted to attain these objects in that it is a compact and lightweight mechanical movement, employs a single small high-speed solenoid triggering element which does little work, and is constructed simply and with the purpose of reducing fatigue and the number of wearing parts.

In brief, this invention includes a continuous driving means which transmits cyclical motion to a driven means to thereby step a pawl and ratchet wheel combination. A rockable interposer is intermittently actuated by a solenoid to link the driving means to the driven means. The pivot points of the pawl and the interposer are radial from and unitarily oscillatable about the ratchet "ice wheel. During the latter portion of each cycle in which the ratchet wheel is stepped, the pawl passes back over a tooth of the ratchet wheel and rocks the intcrposer back into a nonlinking position.

Other objects and features of this invention will become apparent by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a side elevation of an embodiment of this invention shown partly in section and broken away, in the driving position; and

FIG. 2 is an exploded perspective view of a major portion of the invention positioned in the neutral or nondriving position.

In the preferred embodiment of this invention as illustrated in FIGS. 1 and 2, a unidirectional continuously rotating drive shaft 11 is employed as the power input.

Secured to the drive shaft 11 are a pair of drive cams 13 and a pair of complementary drive cams 15 which are angularly displaced from the drive cams 13. Each drive cam 13 is juxtaposed to a complementary drive cam 15 to form a pair of similar spaced apart driving units operable in unison.

Associated with each driving unit is a bifurcated cam follower 17 having an upper arm 19 cooperable with a drive cam 13 and a lower arm 21 cooperable with a complementary cam 15. Each arm of a cam follower is provided with a roller 23, the position of which, as shown in FIG. 2, enables direct contact only with the cam which the particular arm follows.

The cam followers are rotatably mounted on a stepping shaft 25 upon which they are positively driven throughout an oscillatory cycle by the cammed driving units. Each cam follower has a stem portion 27 projecting outwardly from the stepping shaft 25 opposite the follower arms.

A crossmember 2h is secured to the ends of the stem portions 27 and by joining them provides a fixed spacial alignment and parallel relationship between the two cam followers. As shown in FIG. 1, this crossmember has an inverted L shape, which is continued into the stem portions 27 by recesses therein, forming two projecting ledges 31 and 33, the purposes of which will be pointed out hereinafter. Mounted in this manner, the cam followers 17 are free to oscillate about the stepping shaft 25, when impelled by the input from the continuously rotating drive shaft 11, without imparting any motion to the stepping shaft. During such oscillations the ledge 33 of the crossmember 29 will move through an arc area A, as shown in both figures.

A ratchet wheel 35 is secured to the stepping shaft between the two cam followers 17. Loosely mounted on the stepping shaft, adjacent the outer side of each of the cam followers, are a pair of bell cranks 37. One arm 39 of each bell crank is proximate the lower arm 21 of its associated cam follower, and the other arm 41 of each bell crank extends outwardly below the stern portion 27 of its associated cam follower.

Lugs 43 extend perpendicularly from the inner side of each of the bell crank arms 39 and protrude beneath the lower arm 21 of the associated cam follower 17. Oriented in this manner, a clockwise pivoting of the cam followers, as shown in FIG. 2, will cause the lower arms 21 to come into contact with the lugs 43 and urge the bell cranks 37 to translate clockwise about the stepping shaft 25, while imparting no motion to it.

A fixed spindle 47 extends transversely between the arms 41 and maintains their spacial relationship. A driving pawl 49 is pivotally mounted on the spindle 47 near its midpoint and is aligned with the ratchet wheel 35. Also extending between the arms 41 is a fixed rod latching position.

The interposer, pawl, and ratchet wheel all lie substanthe stepping shaft will translatethe pawl finger 57 counterclockwise against the leading face of a tooth61 to cause the ratchet wheel to advance one step. 7 Conversely, clockwise pivoting of the bell cranks '37 will pull the pawl finger 57 back over the trailing face of a tooth 61 of the ratchet wheel 35, thereby rocking the pawl 49 clockwise about the :spindle' 47. The interaction of the pawl finger 57 and the teeth of the ratchet wheel will thenhold the bell cranks 37 in their clockwise position until they are again pivotally driven counterclockwise.

A limit pin lies in the arc of movement of the pawl finger 57 in order to prevent the possible overriding of the pawl and thereby the advancing ofthe stepping. shaft more than one step.

The lower portion of the pawl 49 has a substantially semicircular camming surface 63 adapted to interact with a lip 65 located near the bottom of theinterposer 53.

lowers 17 to pivotcounterclockwise.

. the stepping shaft.

Clockwise rocking of the pawl 49 around the spindle 47 will cause the camming surface 63 to moveagainst the lip 65 and to force the interposer counterclockwise around the rod 51. FIGS. 1 and 2 show the interposers position respectively before and after being ,camrned. To increase smoothness of operation and reduce wear, the semicircular camming surface 63 may be replaced by a small roller.

A solenoid 67 is mounted near the interposer head 59. The solenoid clapper 69 has a catch 71 which is aligned above and has a common plane of motion with noid 67 is not energized, the catch .71 seats into a shoulder 73 in the interposer head 59 and thus holds the interposer head counterclockwise out .ofthe arc area A of the lower ledge33. of the crossmember. 29 in opposition to the biasof the spring 55. The solenoid 67 "can ,be energized intermittently; i.e., either periodically or aperiodically, by a very short pulse from a control means, not shown. When the solenoid is energized, the clapper. 69 is pulled up. out of contact'with the interposer shoulder and the interposeris urgedclockwise by the spring such that the interposer head 59 is moved into the arc area A of the lower ledge 33. '7 I To insure the rapid and accurate seating of the catch 71 into the shoulder 73, a coil spring 75 biases the clap per away from theenergized position and toward the Also, as shown in FIG. 1,.the upper ledge 31 of the cross'member 29 extends over the catch such that during thecounterclockwise stroke of an voscillation of the 'cam followers 17 the ledge 31 will strike latched up in the energized position. 1

FIG. 1 shows the apparatus subsequent to the enerof which thecurrent position of the cams Throughout the operation of this mechanism, power is being applied to the drive shaft 11 to cause it to rotate continuously in one direction at a high rate of speed. This motion is transmitted by the earns 13 and '15 via the rollers 23 to the cam follower arms 19 and 21 which causes the cam followers to oscillate continuously about the stepping shaft25 at a rate twice that of the r.p.m. of the drive shaft. I As shown in FIG. 2, the apices of the complementary cams 15 are impinging upon the rollers of the lower arms 21. This positions the cam followers into their most clockwise location and against the extending lugs 43 of the bell cranks 37, which are also in their most clockwise location. This condition of the bell cranks resulted from the last stepping cycle may or may not have been a part. 1 a V As the cams rotate fromxthe illustrated position, the throw of the driving cams 13 against the upper arms 19 becomes greater than that of the complementary cams against the lower arms 21. .This causes the cam fol- Since, as illustrated in FIG. 2, there is no direct linkage between the stems of the cam followers and the combination of the bell cranks, pawl, and ratchet wheelunlcss the solenoid is pulsed, the combination is not set into motion by the counterclockwise pivoting cam followers to thereby drive Also, since the pawl finger 57 is engaged with the ratchet tooth 61, and the interposer head 59 is biased against the clapper catch 71 which acts as a detent, the loosely mounted bell cranks are not .free to move in a counterclockwise direction. a

Hence, continuously applied input power causes the cam followers to oscillate continuously and freely about the stepping shaft and, assuming the interposer is latched, the bell cranks, previously positioned into their counterclockwise location, will be motionless as willthe stepping shaft. It will also be noted that the lower ledge 33 of the crossmem-ber 29 will be moving through the arc area the interposer 53. AS Shown in FIG 2, when the So1e r A, which causes this element to pass in front of the head 59 of the retracted interposer 53. Additionally, the upper ledge 31 will follow a path which'is aligned with and only slightly above the clapper 69 in its interposer latching position. i v Y A two-phase stepping cycle 'is commenced by the en- .ergization'of the solenoid. At such time (subsequent to the view of FIG. 2, and prior to the view of FIG. 1),

a the clapper 69 is pulled up and the interposer head 59 is released to follow the clockwise urging of the torsion spring55. Assuming-that the energization pulse is congization pulse of the solenoid and during that portion of the cycle when the ratchetwhecl is being stepped 'while the clapper of the then deenergized solenoid is being knocked down by the upper ledge 31.

, A pawl 77, a spring 79, and their associated mounting means provide an ,antibackup. assembly-which prevent imparted to the stepping shaft 25 since the interposer 53 is held in its counterclockwise, nonlinking position by the solenoid clapper catch 71. w I

te'mporarieous with the existence of a near maximum arc area A, the condition shown in FIG. 2, the interposer head 59 will be moved directly beneath the lower ledge 33 of the crossmember 29 untilthe shoulder 73 isstopped by the crossmember, as shown in FIG. 1. As the cam followers proceed through that stroke of their oscillation that would normally reduce the arc area A from.maximum to minimum, the condition shown in FIG. 1, the ledge 33 will abut the interposer head '59 adjacent the shoulder 73 and act as a driving arm 'to force the interposer downward- This downward force is transmitted from theinterposer. to the bell cranks via the transverse rod 51, causing the bell cranks to translate counterclockwise about the stepping shaft throughan arc substantially equal'to that subtended by the cam follower stems The translating of the bell cranks causesthe spindle 47 to be displaced counterclockwise and thereby also carries the'pawl 49 in that direction, Since the pawl finger 57 is biased intotheteeth ofthe? ratchet wheelftby the torsion spring 55, its displacement will drive it against the tooth 61 to cause theratchet wheel to stepv counterclockwise one tooth position.

As previously stated, the'solenoid is energized by a short pulse from a control means. Hence, by the time that the interposer head is positioned beneath the crossmember 29, the solenoid is again deenergized. The clapper is then free to drop down under the influence of coil spring '75 into the position shown in FIG. 1. If residual magnetism continues to hold up the clapper, the upper ledge 31 of the crossmember 29 will knock it free. Thus, the clapper is again in position to latch the interposer during the next clockwise excursion of the bell cranks 37.

During the second and final phase of the stepping cycle, the complementary cams l drive the cam followers back to their initial clockwise position. During the latter rotation the lower arms 21 impinge upon the lugs 43 on the bell crank arms 39 and thereby cause the repositioning of the bell cranks to their initial clockwise location.

As the bell cranks 37 translate clockwise, the driving pawl 49, which is pivotally mounted on the transverse rod 51 between the bell crank arms 41, tends to backstep the ratchet wheel; however, the antibackup pawl '77 immobilizes the ratchet wheel. Since the driving pawl 49 is being displaced clockwise and its finger 57 is meshed in the teeth of the immobile ratchet wheel, the pawl finger is forced to disengage itself from between the ratchet teeth by camming upon the trailing face of a tooth. This action forces the pawl 49 to rock clockwise in opposition to the bias of the spring 55. The rocking of the pawl causes its camming surface 63 to impinge upon the lip 65 of the interposer and thereby forces the entire interposer sufficiently counterclockwise so that its shoulder 73 is positioned to the left of the clapper catch 73;. Just as the shoulder 73 is raised to the latching level by the clockwise translation of the transverse rod 51, the pawl finger 57 slips into mesh with the next ratchet tooth 61, the

trailing face of which the pawl finger had just cammed upon, and is rocked back into its normal counterclockwise position by the spring 55. Upon the return of the pawl to its normal position, the camming surface 63 retracts from the interposer lip 65 and permits the interposer to be carried to the right by the torsion spring 55 until the shoulder 73 is latched by the catch 71.

The above completes the description of one full operating cycle during which the stepping shaft has been rotated through one incremental step in response to a single periodic or aperiodic energization of the solenoid. The driving shaft 11 has made one-half of a revolution and has produced a single oscillation of the cam followers. If the drive shaft were operating at a conventional 1800 rpm, and if the solenoid were energized periodically during each cycle by the control means, the stepping shaft would be incrementally advanced 60 times a second. Hence a ledger sheet or tape could be fed at the rapid rate of 60 linear increments a second.

Should the periodic mode of stepping not be desired for any portion of the operation of this invention, an

aperiodic energization from the control means could be employed. This mode of advancing the stepping shaft could be under random manual control or control by a programming device. A combination of periodic and aperiodic, i.e., intermittent stepping could also be accomplished by this device if the energization were so applied.

Upon energization of the solenoid, the only work that it must accomplish is the lifting of its clapper while overcoming the force applied by the torsion spring 55 which urges the interposer shoulder against the clapper catch. Since this is a very negligible amount of work, a very lightweight, fast make and break solenoid may be successfully employed.

It may be further noted that a simple continuous mode of operation can be obtained from this invention by the removal of the solenoid assembly and the control means. In this arrangement the interposer head will be biased into and cammed out of the path of the crossmember 2? during each cycle and the stepping shaft will respond similarly to its periodic mode. Of course, assuming a continuous input, an aperiodic operation is not possible unless the interposer can be latched out.

From the above description it will be apparent that this invention can operate successfully at a high rate of speed while employing a relatively few simple and sturdy components. It should also be noted that the cam followers or their equivalents could be set into continuous cyclical motion; i.e., oscillatory or reciprocatory motion by known means other than those described and thereby cyclically drive the bell cranks or their equivalents.

More fundamentally, the driving element 33 could be made cyclical by any known means, it being required only that the arm 41 be rocked clockwise during the nondriving portion of the cycle of the element 33.

Additional changes and substitutions in the form and details of this invention could also be made by those skilled in the art without departing from its fundamental novel features. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A rapid, intermittent stepping movement comprising:

a stepping shaft,

a unidirectional ratchet wheel secured to said stepping shaft,

a pair of spaced apart cam followers oscillatable about said stepping shaft, each of said cam followers having a bifurcated end and a driving end,

a crossmember secured to said driving ends and having a first and a second impelling portion,

a continuous, unidirectional rotary input means having two similar pairs of spaced apart parallel cams thereon, one pair associated with each of said cam followers,

said cams of each of said pairs being angularly displaced With respect to each other and impingeable upon a different one of said bifurcations thereby providing a continuous, driven oscillation of said cam followers by said input means,

a pair of hell cranks translatable about said stepping shaft, each associated with an individual one of said cam followers, the first arm of each of said bell cranks being proximate one of said bifurcations and the second arm being driven in a first direction by said first impelling portion,

a lug on each of said first arms impingeable by its associated bifurcation during one portion of an oscillation to translate said bell cranks in a second direction,

a ratchet pawl biased into the teeth of said ratchet wheel and having a camming surface thereon,

an interposer biased in one direction into the path of said first impelling portion of said crossmember and cammable in the other direction by said camming surface of said ratchet pawl,

said interposer and said ratchet pawl being pivotally mounted on said second arms of said bell cranks andtranslatable therewith, and

an intermittently actuable, electromechanical detent for restraining said interposer and lying in the path of said second impelling portion of said crossmember,

whereby translation of said bell cranks in said first direction by said first impelling portion of said crossmember steps said ratchet wheel through said ratchet pawl, and translation of said bell cranks in said second direction causes said ratchet pawl to pivot upon a tooth of said ratchet wheel and thereby cam said interposer against said bias out of the path of said first impelling portion and into the control of said detent.

2. In an intermittent stepping movement having an incremental unidirectional rotary output and utilizing a continuously oscillating input with a driving motion and a nondriving motion,

a pivoted pawl actuating said rotary output, and

' saidfratchet pawl translatable said pivoted pawl has a camming surface interacting with said interposer,

said'rotary output 1s. a ratchet'wheel, and r V a said pawl and said interposer are also translated cir cumferentially in unison by the nondriving motion of said input about the center of said ratchet wheel in the direction opposite to said rotary output, whereby the passing of said pawl over a tooth of said ratchet wheel pivots said pawl and thereby cams said interposer out of the path of the driving motion of said oscillating input. Q a

4. The combination of claim 3 including 1 a solenoid controlling said interposer and having a clapper with a detent 7. surface thereon for latching said interposer out of the pathof said driving motion when said solenoid is in its deenergized state..

5. A controllable stepping movement comprising:

a stepping shaft, 7 i

a ratchet wheel secured to said stepping shaft,

a continuously cyclical driving element loosely mountedon said stepping shaft and having a first portion andasecond portion, I

-an interposer controllably positionable intov the'path of said first portion of. said driving element;

a ratchet pawl pivota'lly, mountedladjacent said inter-2 poser' and translatable thereby in one direction to drive said ratchet wheel,- 7

through said second portion of said driving element and being thereupon cammed by the teeth of said ratchet whee1,.and

an interposer repositioning portion on said ratchet pawl, a a 7 whereby said ratchetwheel'is stepped by said first portion through said interposer' and said pawl, arid said interposer is repositioned out of the path of said first portion by said pawl when said pawl is translated by said second portion. 1

i 6. In a'stepping movement having a continuously cyclical input driver and an on-demand ratchet wheel output,

an interposer pivotable into and out of the path of said input driver, a 1 v a a rockable ratchet pawl, ra a commonmounting for said-interposer and said pawl oscillatable about said ratchet wheel,

the teeth of saiduratchet wheel causing said pawl to rock during a portion of an oscillation of said mounta releasable detent for "restraining said interposer out of the path of said input driver, and 7 means on said pawl for pivoting said interposer into the control of said detent when said pawlis rocked.

7. An intermittent stepping'movement comprising:

a continuous cyclical driving means,

a driven means cyclically drivable through said driving ,means, t t 7 said driving means and said driven means each having a first arm and'a second arm, respectively adjacent one? .an interposer pivoted on said second arm of said driven means having a surfacepositionable to be driven by said second arm of said driving means, a unidirectional ratchet wheel,

(a ratchet pawl rockable .011 said second arm of said in. another direction I driven means and having a ratchet wheel impelling portion and an interposer repositioning portion, means biasing said surface of said interposer into said position to be driven and also biasing said impelling portion of said ratchet pawl into the teeth of said ratchet wheel, f an intermittently actuatable detent selectably releasing said interposer into the control of said biasing means during another portion of said cycle, whereby said ratchet wheel is stepped'by saidpawl through the interactionof said second arms of said driving and driven means, and said interposer, said first arm of said driven means, when driven, translating said second arm of said driven means toward said detent, rocking said pawl over a tooth of said ratchet wheel, and thereby causing said interposer repositioning portion of saidpawl to coact with said interposer and thereby to reposition said interposer into the control of said detent. v 8. The stepping movement of claim 7 including f a cammed input means coupled to said first arm of said driving means, and v wherein said first arm of said driving means is a bifurcated cam follower, said driven means is a bell crank,

' are loosely mounted on an axis common'with said ratchet wheel. 7 9. The stepping movement of claim 8 wherein said interposer,'said pawl, said ratchet wheel, and said 0 detent have a common plane of motion.

' 10. A'steppin'g movement according to claim 7 wheresaid'detent is a clapper of a fast, action solenoid, c said driving means comprises a first pair of similar spaced apart elements, said driven means comprises a spaced-apart'elements, and V a all of said elements are maintained in parallel relationship. 11. In a mechanism having acontinuously operating, cyclical driving means, a unidirectional, rotary, output means, and means for advancing said output means incrementally,'said advancing means beingcammed by said output means while being'repositioned for a further advance of said output means,

a member drivable by said cyclical driving means an advancing phase and a repositioning phase, an interposer positionable to be actuated by said driving means, said interposer and said advancing means bothrbeing pivotally mounted on said drivable member, means on said advancing means for camming said interposer while said advancing means is being cammed by said output means, whereby, during said advancing phase, said output means is advanced through said drivable member and said actuated interposer and, during said repositioning phase, said, advancing means is repositioned andpsaid interposer is cammed out ofposition to be actuated by said driving means. 12. The mechanism according to claim 11 including means biasing said interposer into position to be actuated by said driving means, and

controllable latching means for restraining said interinto poser against said bias, 7

said camming of said interposer during said repositioning phase placing said interposer under the restraint of said latch and thereby also restraining said 'drivable member.

References Cited by the Examiner UNITED STATES PATENTS 3,139,763 7/64 Lisinski 74125 7 BROUGHTON G. DURHAM, Primary Examiner.

and both said driving means and said driven means second pair of similar 

1. A RAPID, INTERMITTENT STEPPING MOVEMENT COMPRISING: A STEPPING SHAFT, A UNIDIRECTIONAL RATCHET WHEEL SECURED TO SAID STEPPING SHAFT, A PAIR OF SPACED APART CAM FOLLOWERS OSCILLATABLE ABOUT SAID STEPPING SHAFT, EACH OF SAID CAM FOLLOWERS HAVING A BIFURCATED END AND A DRIVING END, A CROSSMEMBER SECURED TO SAID DRIVING ENDS AND HAVING A FIRST AND A SECOND IMPELLING PORTION, A CONTINUOUS, UNIDIRECTIONAL ROTARY INPUT MEANS HAVING TWO SIMILAR PAIR OF SPACED APART PARALLEL CAMS THEREON, ONE PAIR ASSOCIATED WITH EACH OF SAID CAM FOLLOWERS, SAID CAMS ON EACH OF SAID PAIRS BEING ANGULARLY DISPLACED WITH RESPECT TO EACH OTHER AND IMPINGEABLE UPON A DIFFERENT ONE OF SID BIFURCATIONS THEREBY PROVIDING A CONTINUOUS, DRIVEN OSCILLATION OF SAID CAM FOLLOWERS BY SAID INPUT MEANS, A PAIR OF BELL CRANKS TRANSLATABLE ABOUT SAID STEPPING SHAFT, EACH ASSOCIATED WITH AN INDIVIDUAL ONE OF SAID CAM FOLLOWERS, THE FIRST ARM OF EACH OF SAID BELL CRANKS BEING PROXIMATE ONE OF SAID BIFURCATIONS AND THE SECOND ARM BEING DRIVEN IN A FIRST DIRECTION BY SAID FIRST IMPELLING PORTION, A LUG ON EACH OF SAID FIRST ARMS IMPINGEABLE BY ITSE ASSOCIATED BIFURCATION DURING ONE PORTION OF AN OSCILLATION TO TRANSLATE SAID BELL CRANKS IN A SECOND DIRECTION, A RATCHET PAWL BIASED INTO THE TEETH OF SAID RATCHET WHEEL AND HAVING A CAMMING SURFACE THEREON, AN INTERPOSER BIASED IN ONE DIRECTION INTO THE PATH OF SAID FIRST IMPELLING PORTION OF SAID CROSSMEMBER AND CAMMABLE IN THE OTHER DIRECTION BY SAID CAMMING SURFACE OF SAID RATCHET PAWL, SAID INTERPOSER AND SAID RATCHET PAWL BEING PIVOTALLY MOUNTED ON SAID SECOND ARMS OF SAID BELL CRANKS AND TRANSLATABLE THEREWITH, AND AN INTERMITTENTLY ACTUABLE, ELECTROMECHANICAL DETENT FOR RESTRAINING SAID INTERPOSER AND LYING IN THE PATH OF SAID SECOND IMPELLING PORTION OF SAID CROSSMEMBER, WHEREBY TRANSLATION OF SAID BELL CRANKS IN SAID FIRST DIRECTION BY SAID FIRST IMPELLING PORTION OF SAID CROSSMEMBER STEPS SAID RATCHET WHEEL THROUGH SAID RATCHET PAWL, AND TRANSLATION OF SAID BELL CRANKS IN SAID SECOND DIRECTION CAUSES SAID RATCHET PAWL TO PIVOT UPON A TOOTH OF SAID RATCHET WHEEL AND THEREBY CAM SAID INTERPOSER AGAINST SAID BIAS OUT OF THE PATH OF SAID FIRST IMPELLING PORTION AND INTO THE CONTROL OF SAID DETENT. 